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170 Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 significant saving is if the biogas is used for CHP and highly efficient use of the heat generated is achieved. 11.2 Scenario 2: Wood Waste Wood waste is taken here to be wood that is contaminated, either impregnated with preservatives, or coated with paints and varnished. Disposal therefore requires WID compliant facilities, and currently only the largest of facilities have appropriate emissions cleaning facilities. While a considerable quantity of such material is estimated to be disposed of each year, there is little good quality data for this waste stream. Estimations from WRAP put the volume of wood waste at 1,065,000 tonnes from MSW, 602,000 tonnes from waste furniture, 4,400,000 tonnes from C&D waste (5,040,000 if include reclaimed wood), and 4,481,000 tonnes from C&I waste. This would potentially offer 10,548,000 tonnes of wood waste a year. The options available for the disposal of wood waste are for it to be sent to landfill or for it to be segregated and used as a feedstock for combustion, generating heat, CHP or electricity. An alternative possibility is for a conversion process to be used, such as gasification followed by Fischer Tropsch to generate a transport fuel. This however hasn’t been modelled as we have been unable to identify sufficient data on using wood waste as a feedstock for this process. Waste Wood Combustion Landfill With landfill gas Heat, CHP or Electricity Figure 24 Disposal options for waste wood It is assumed that most wood waste is disposed of to landfill with energy recovery. Such a scenario has not been taken as the reference system for this work as the intention here is to compare the different end uses of waste wood, and comparison to landfilling can generates large negatives that do not allow accurate comparison. In the following situations the GHG savings come from the displaced fossil fuel, in each case natural gas, so once again giving a conservative estimate of the savings generated. Two different scenarios considering the GHG saving that could be achieved from using waste wood as a feedstock for energy generation. 1. Industrial or commercial building heating. This system is already economic, but still has challenges to face including WID compliant facilities, fuel storage and ash disposal. 2. Combustion in a medium scale plant – the practical limit for waste levels generated within an RDA geographical region. Such a system would be economic with the currently available subsidy i.e. the availability of 1.5 ROC. Both systems have been considered in terms of the displacement of natural gas as the fossil fuel equivalent and are estimated to would produce the following GHG savings:
Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy AEA/ED45551/Issue 1 Table 87 Combustion of wood waste and the savings generated from avoided fossil fuel consumption End Use GHG savings (Kg CO2eq / tonne of biomass) • Industrial or Commercial building heating • 500kW boiler using wood chips 1,040 • Operating at 33% load • Medium combustion plant • 50MWe exported 450 • 85% load. Significant GHG savings would be achieved were wood waste to be used as a fuel source for combustion, rather than disposal to landfill. 11.3 Other, recent GHG life cycle analyses Recent work by the Environment Agency has analysed a wide range of biomass energy cycles using the BEAT tool. Such cycles take in to account the fate of by-products and the production of the fuel. The main findings of this work are that • Heat applications are the most beneficial in terms of GHG abatement followed by CHP where the heat load is high. • Transport fuel production by means of gasification is mid range. • Electricity production comes lower in the hierarchy as do biological transport fuel processes and low heat load CHP. What is interesting from this work is that it is the resource use efficiency to energy that is the determining factor, so that heat will always score highly because it converts over 90% of the energy of the feedstock into usable heat. The performance of CHP is very dependent on the heat load; industrial process heat loads score well but minimal application of district heat scores badly. Given the high resource use efficiency gasification and methanation should be very beneficial if the high grade heat from the exothermal reaction is fully used, however there is no data on these systems at present. 171
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